Solid-state image sensors and cameras find more and more applications in areas where it is difficult to focus the incident electromagnetic or particle radiation with optical means. This is particularly true for X-rays, gamma rays, high-energy ultraviolet light (with wavelengths below 100 nm) and neutral beams of atoms. At the same time, the absolute level of incident radiation intensity is often very low, making it desirable to acquire images of this incident radiation with pixels that are at the same time highly sensitive and quite large. Such large pixels should have an area of 25 square microns or much more, e.g., up to several square centimeters, which could be of interest in X-ray applications.
The image-sensor pixels known from literature are either of the photodiode or the metal-oxide-semiconductor (MOS) device type, as described for example in P. Seitz, “Solid-State Image Sensing”, in Computer Vision and Applications—A Guide for Students and Practitioners (B. Jahne and H. Haussecker, Eds.), pp. 111-151, Academic Press, San Diego, 2000. In such photosensors, the photocharge-detection sensitivity varies with the inverse of the pixel's capacitance. This capacitance, on the other hand, increases in direct proportion with the pixel area. For this reason, it is not possible to realize such conventional photosensors that are at the same time very large, highly sensitive and fast.
A first method to overcome this contradiction is taught in U.S. Pat. No. 4,245,233. An MOS structure is described, consisting of a highly resistive layer on top of an insulator covering a semiconductor. A voltage difference is applied to the two ends of the highly resistive layer, creating a spatially varying potential distribution at the interface between semiconductor and insulator. Incoming photons generate charges in the bulk of the semiconductor, and the photogenerated charges move to the semiconductor interface essentially by diffusion. Once they are close to the semiconductor-oxide interface, they notice the spatially varying surface potential, and they move along the electric field lines to the region with the attracting potential energy, at one end of the device. At this place a diffusion at the semiconductor surface is employed to collect the photocurrent, making use of a transimpedance circuit that keeps the diffusion at a fixed potential. Since this type of photosensor makes use of a transimpedance circuit, the complete device covers a large area. Therefore, it is only useful, in practice, for single photodetectors or, at most, for a linear array of photodetectors that offer at the same time large areas and high charge-detection sensitivity.
A method that allows the realization of two-dimensional arrays of large-area, sensitive pixels is described by P. Seitz et al. in “Smart optical and image sensors fabricated with industrial CMOS/CCD semiconductor processes”, Proc. SPIE, Vol. 1900, 21-30, 1993. The so-called “charge-binning” method makes use of standard CCD technology and employs a special charge transport/accumulation technique. The CCD gates are clocked such that charge packets from different pixels are accumulated under one gate. Thus, this summed charge can be read out instead of reading out all pixel charge packets individually. In a two-dimensional CCD image sensor it is possible to employ this charge-binning method to realize two-dimensional areas of an effective photosensitive size (“super pixels”) that is much larger than the one of individual pixels, and these super pixels can even have non-rectangular shape. However, this implies the use of industry-standard CCD technology for the fabrication of CCD image sensors, as well as suitable CCD clocking circuitry and schemes with the associated system complexity and high electric power consumption.
U.S. Pat. No. 5,528,643 describes the fast lateral transport of photogenerated charge carriers, by employing a series of CCD gates, each of which has contacts at both ends at which voltage differences can be applied. In this way, each CCD electrode exhibits a lateral drift field at the semiconductor-insulator interface. The object of invention is the architecture of a two-dimensional CCD image sensor with improved photocharge transport speed in the column and read-out line directions. As in the charge-binning approach described above, the teaching of said patent necessitates CCD clocking circuitry and clocking schemes. Again, system complexity and power consumption are rather high.
An alternate structure without clocked electrodes is taught in U.S. Pat. No. 4,885,620, describing so-called “drift detectors”, which are especially used for the detection of particles and ionizing radiation. Because of the large penetration depth of this form of incident radiation, the detectors must have three-dimensional structures. They produce a lateral drift field in the center of a fully depleted wafer using electrodes and floating implants on both sides (top and bottom) of the wafer. An impinging particle creates an electron-hole cloud along its trajectory. These charge-carriers then drift sideways along the lateral electric field and are read out at the side electrode which has the highest or lowest potential, respectively. From the time between the particle impact and the arrival of the charge carriers at the read-out node one can calculate the lateral position of the particle's trajectory. Such devices are dedicated to the measurement of spatial coordinates of particle trajectories and not to demodulation purposes. Furthermore, they need a double-sided wafer-processing and the application of high voltages to fully deplete the wafer, making it impossible to fabricate them with industry-standard CMOS or related semiconductor processes.
The publication WO-2004/001354 discloses an image-sensing device and a method for detecting and demodulating modulated wavefieds. Each pixel consists of a resistive, transparent electrode on top of an insulated layer that is produced over a semiconducting substrate whose surface is electrically kept in depletion. The electrode is connected with two or more contacts to a number of clock voltages that are operated synchronously with the frequency of the modulated wave field. In the electrode and in the semiconducting substrate, lateral electric fields are created that separate and transport photogenerated charge pairs in the semiconductor to respective diffusions close to the contacts. By repetitively storing and accumulating photocharges in the diffusions, electrical signals are generated that are subsequently read out for the determination of local phase shift, amplitude and offset of the modulated wave field. This device also consumes large amounts of electric power.
EP-0,862,226 A2 describes planar structures for the detection of electromagnetic and particle radiation. In contrast to CCD structures, where an array of gate electrodes is placed on an insulating layer, it makes use of an array of diffusions in a semiconductor substrate for the lateral charge transport. In both cases, however, each gate or diffusion electrode must be wired individually, and it is connected to its own, particular voltage source. According to EP-0,862,226 A2, the preferred means for generating the different voltages is a voltage divider crated with a tapped resistor. As a consequence, the generation of these voltages requires the flow of a current and, therefore, the dissipation of electrical power. Furthermore, the silicon wafer bulk has to be fully depleted for an operation of the device. In order to deplete an average wafer thickness of approximately 300 μm, a potential difference in the order of several hundred Volts is needed.
U.S. Pat. No. 4,788,581 describes an MOS dosimeter with floating metal gates. However, the generated charges are collected on the metal gates, not in the substrate. No dump mechanism for quickly reading out the charges is provided, nor is such a mechanism possible in the MOS dosimeter. Therefore, the dosimeter is not suitable for imaging purposes, where the pixels have to be aligned in a large array and where a fast sequence of images is required.